1. Field of the Invention
The invention relates to coating devices and in particular to devices for coating a paper web.
2. Description of Related Technology
The state of the art regarding paper coating devices is described in the following publications:
(1) DE-GM 8 414 413; PA1 (2) U.S. Pat. No. 2,729,192; PA1 (3) U.S. Pat. No. 2,946,307; PA1 (4) U.S. Pat. No. 3,084,663; PA1 (5) U.S. Pat. No. 5,112,653; PA1 (6) EP Patent Application 93 112 695; and PA1 (7) EP 507 218 A1 (corresponding to U.S. Pat. No. 5,230,165).
With reference to FIG. 1 of publication (7), a known coating device has two applicator rolls which form a gap through which a paper web passes. A nozzle applicator station is assigned to each of the two applicator rolls where a coating composition film is applied onto a surface of a respective roll. The nozzle applicator station has a nozzle with two nozzle lips, one of which has a roller blade on a free end thereof. Thus, a metered application of a coating composition onto the surface of a particular roll is possible. Accordingly, such a device provides an indirect method of application of a coating onto paper because the coating is first applied onto a surface of the roll and is then transferred from the roll onto a paper web.
It is also possible to use applicator stations different from the one described in publication (7). Moreover, it may not be necessary to coat both sides of a paper web. Thus, for example, it may be possible to coat only one side of a paper web or to apply coating on both sides of a web but at different rates whereby several roll pairs are provided at an assigned applicator station and a paper web is dried each time it receives a coating.
Such coating devices have proven sufficient in the past. However, a problem with such devices that has not been satisfactorily addressed is known as the orange-peel effect wherein the coating is not flat on the paper web after the paper leaves the roll gap but rather has a surface that is reminiscent of the appearance of an orange peel. Many attempts have been made to eliminate the orange-peel effect, as, for example, by utilizing a smoothing or rubbing element connected to the coating device. Although such a measure makes it possible to ameliorate the undesirable orange-peel effect, it typically has not completely eliminated it. In addition, providing rubbing elements, for example, results in increased equipment cost.